Two-step brazed X-ray target assembly

ABSTRACT

A joining method designed to minimize the temperature needed to obtain a high strength braze joint between a molybdenum alloy substrate and a graphite disk used in a rotating anode X-ray tube target used for computed tomography applications. The method consists of two separate brazing operations. The first brazing operation joins a thin molybdenum sheet to the graphite disk using a pure metal braze to form a plated graphite subassembly. The second brazing operation joins the plated graphite subassembly to the molybdenum alloy substrate using a highly specialized braze alloy having a melt temperature below the recrystallization temperature of said molybdenum alloy substrate and a remelt temperature after brazing above the recrystallization temperature of said molybdenum alloy substrate. This two step brazing reduces the probability of fracture in the graphite by maintaining the elevated temperature yield strength normally developed in forged molybdenum alloy substrates by avoiding the deleterious yield strength reduction associated with recrystallization of the molybdenum alloy substrate.

This application is a division Ser. No. 09/752,190, filed Dec. 29, 2000.

TECHNICAL FIELD

The present invention relates generally to a radiography device and,more particularly, to a two-step brazed x-ray target assembly for aradiography device.

BACKGROUND ART

The X-ray tube has become essential in medical diagnostic imaging,medical therapy, and various medical testing and material analysisindustries. Typical X-ray tubes are built with a rotating anodestructure for the purpose of distributing the heat generated at thefocal spot. The anode is rotated by an induction motor consisting of acylindrical rotor built into a cantilevered axle that supports thedisc-shaped anode target, and an iron stator structure with copperwindings that surrounds the elongated neck of the X-ray tube thatcontains the rotor. The rotor of the rotating anode assembly beingdriven by the stator which surrounds the rotor of the anode assembly isat anodic potential while the stator is referenced electrically to theground. The X-ray tube cathode provides a focused electron beam that isaccelerated across the anode-to-cathode vacuum gap and produces X-raysupon impact with the anode.

In an X-ray tube device with a rotatable anode, the target haspreviously consisted of a disk made of a refractory metal such astungsten, and the X-rays are generated by making the electron beamcollide with this target, while the target is being rotated at highspeed. Rotation of the target is achieved by driving the rotor providedon a support shaft extending from the target. Such an arrangement istypical of rotating X-ray tubes and has remained relatively unchanged inconcept of operation since its induction.

However, the operating conditions for X-ray tubes have changedconsiderably in the last two decades. Due to continuous demands fromradiologists for higher power from X-ray tubes, more and more tubes areusing composite rotating anodes with tungsten-rhenium as a focal spotlayer, molybdenum alloy (typically TZM) as a substrate, and brazedgraphite as a heat sink.

The higher power levels increase the operating temperatures of the anodewhich, if high enough, may result in elevated temperature plastic hoopstrain deformation of the molybdenum alloy substrate. The magnitude ofthe strains increases as the center of the anode is approached. Largehoop strains may induce stress in the metallurgical bond between thealloy substrate and the graphite heat sink. The magnitude of this stressimposes a limit on the maximum size, rotational speed and highestallowable temperature of the alloy substrate. Should the stress exceed athreshold value, a complete debond of the graphite heat sink can result.

The metallurgical bond made between a TZM substrate and the graphiteheat sink is accomplished by elevated temperature brazing, which can beas high as 1900 degrees Celsius. Prior to brazing, the TZM substrate istypically forged to a final shape that greatly enhances the strength ofthe material. However, during the high temperature brazing process, thisstrength increase may be lost due to metallurgical transformation, orrecrystallization, in the TZM, which takes place near or above 1400degrees Celsius.

It would be desirable to have an improved X-ray tube target design whichwould reduce the heat needed in the brazing step to attach a molybdenumalloy substrate cap to the graphite disk to overcome problems associatedwith prior art structures and for improving the power limits of advancedX-ray tubes.

SUMMARY OF THE INVENTION

The present invention provides an improved joining method between amolybdenum alloy substrate cap and a graphite disk used in x-ray tubetargets for computed tomography applications.

Two interrelated brazing operations are used to join the molybdenumalloy cap, typically TZM, to the graphite disk. A first brazing stepjoins a thin molybdenum alloy sheet to the graphite disk using either apure zirconium or pure titanium braze to form a “plated” graphitesubassembly. A second brazing step joins the plated subassembly to themolybdenum alloy substrate cap using a select group of highlyspecialized brazed alloys to form a final assembly.

These highly specialized brazed alloys are designed to have melttemperatures below the recrystallization temperature of the molybdenumalloy substrate (about 1400 degrees Celsius) and a remelt temperatureafter brazing, due to the diffusion of molybdenum into the braze joint,at or near 1700 degrees Celsius. High remelt is critical to fullyexploit the advantage of using a molybdenum alloy substrate for rotatinganode applications. By reducing the temperature that the molybdenumalloy substrate is exposed to in the brazing steps, recrystallization ofthe molybdenum alloy substrate is avoided, resulting in higher yieldstrengths for the molybdenum substrate. These higher yield strengthmolybdenum substrates exhibit lower rotation hoop strains at thesubstrate/graphite interface that reduces the possibility of tubefailure by reducing the possibility of fracture in the graphite disk.

Other objects and advantages of the present invention will becomeapparent upon the following detailed description and appended claims,and upon reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an X-ray tube constructed inaccordance with the prior art;

FIG. 2 is a cross-sectional view of an X-ray tube constructed inaccordance with the present invention;

FIG. 3 is an exploded view of the anode target assembly of FIG. 2; and

FIG. 4 is a logic flow diagram for forming the anode target assembly ofFIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following figures, the same reference numerals are used toidentify the same components. The present invention relates to acomputed tomography system having rotating X-ray tubes that employ arotating anode system and a cathode assembly.

Referring now to the drawings, FIG. 1 illustrates an X-ray tube anode 10having a rotating anode assembly 12 according to the prior art. Theanode assembly 12 is rotated by a bearing shaft 20, which supports adisc shaped target 14, typically comprising a tungsten-rhenium area 18for generating X-rays, a molybdenum alloy substrate 48 for structuralsupport, and a graphite disk 16 operating as a heat sink. The target 14is connected via the stem 24 to the bearing shaft 20, which is supportedby bearings 22 facilitating rotation. The graphite disk 16 is joined tothe molybdenum alloy substrate 48 by using a braze alloy (not shown)consisting of either pure titanium, pure zirconium, or alloys withtitanium and zirconium as a base. The end of bearing shaft 20 isattached to a rotor (not shown) driven by a stator (not shown) of aninduction motor (not shown). The entire rotating assembly 12 is atanodic potential while the stator is referenced electrically to ground.

In a typical assembly, the rotating anode assembly 12 and a cathodeassembly (not shown) are sealed in a vacuum envelope (either glass orbrazed metal construction) and mounted in a conductive metal housing(not shown). The rotating anode assembly 12, the stator, and the vacuumenvelope are insulated electrically from each other. A typical X-raytube anode 10 further comprises a X-ray tube cathode assembly (notshown) for providing a focused electron beam that is accelerated acrossa large anode-to-cathode vacuum gap, thereby producing X-rays uponimpact with the anode.

One problem with X-ray tube anodes 10 in the prior art is that thetemperature needed to obtain a high strength braze joint between themolybdenum alloy substrate 48 and graphite disk 16 is higher than therecrystallization temperature of the molybdenum alloy substrate 48.Recrystallization decreases the elevated temperature yield strength ofthe molybdenum alloy substrate 48 that is imparted into the molybdenumalloy substrate 48 during forging fabrication. The elevated temperatureyield strength is a physical property that places a limit on the X-raypower generated by the tube. A higher yield strength enables a higherrotational speed, anode size, or operating temperature, or a combinationof all three.

The present invention proposes a method for minimizing the temperatureneeded to obtain a high strength braze joint between the molybdenumalloy substrate 48 and the graphite disk 16 and thus avoid thedeleterious strength consequences of recrystallization.

As shown in FIG. 2, an X-ray tube anode 35 having a rotating anodeassembly 12 according to the present invention is shown. The X-ray tubeanode 35 is rotated by a bearing shaft 20, which supports a disk shapedtarget 38. The target 38 has a tungsten-rhenium area 18 for generatingX-rays, a molybdenum alloy substrate 48 for structural support, and agraphite disk 42 as a heat sink. The target 38 is connected via the stem24 to the bearing shaft 20, which is supported by bearings 22facilitating rotation. The target 38 is typically welded to the stem 24using a brazed alloy. Alternatively, the target 38 may be bolted to thestem 24. The end of bearing shaft 20 is attached to a rotor (not shown)driven by a stator (not shown) of an induction motor (not shown). Theentire rotating assembly 12 is at anodic potential while the stator isreferenced electrically to ground. A close-up view for coupling themolybdenum alloy substrate 48 to the graphite disk 42 to form a anodetarget assembly 60 is shown below in FIG. 3, and a description of theprocess for making the anode target assembly 60 is shown in FIG. 4.

As in the assembly of FIG. 1, the rotating anode assembly 12 and acathode assembly (not shown) are sealed in a vacuum envelope (eitherglass or brazed metal construction) and mounted in a conductive metalhousing (not shown). The rotating anode assembly 12, the stator, and thevacuum envelope are insulated electrically from each other. A typicalX-ray tube anode 35 further comprises a X-ray tube cathode assembly (notshown) for providing a focused electron beam that is accelerated acrossa large anode-to-cathode vacuum gap and producing X-rays upon impactwith the anode.

FIG. 3 is a close-up view of the anode target assembly 60 of FIG. 2.Referring now to FIGS. 2 and 3, a first brazing step joins a thinmolybdenum alloy sheet 40 to a graphite disk 42 using either a purezirconium or pure titanium braze 44 to form a plated graphitesubassembly 46. Preferably, the thin molybdenum alloy sheet 40 isapproximately 0.5 mm thick. A second brazing step joins the subassembly46 to a molybdenum alloy substrate 48 using a select group of highlyspecialized brazed alloys 50 to form the anode target assembly 60.

These highly specialized brazed alloys 50 are designed to have melttemperatures below the recrystallization temperature of the molybdenumalloy substrate 48 and remelt temperatures above the recrystallizationtemperature of the molybdenum alloy substrate. Two alloy brazes 50 arepreferred. The first, hereinafter referred to as Ti—Cr—Be braze, is amixture containing approximately 72 parts by weight titanium, 25 partsby weight chromium, and 3 parts by weight beryllium, and has a liquidusof approximately 1110 degrees Celsius. The second, hereinafter referredto as Ti—Si braze, is a mixture containing approximately 91.5 parts byweight titanium 8.5 parts by weight silicon, wherein the mixture has aliquidus of approximately 1200 degrees Celsius. After brazing, theremelt temperature for both of the alloy brazes 50 is approximately 1700degrees Celsius due to the diffusion of molybdenum from the molybdenumalloy substrate 48 to the braze 50.

By reducing the temperature that the molybdenum alloy substrate 48 isexposed to in the brazing steps below approximately 1200 degreesCelsius, which is the temperature at which the molybdenum alloy yieldstrength breaks down, higher yield strengths for the molybdenum alloysubstrate 48 are obtained. These higher elevated temperature yieldstrength molybdenum alloy substrates 48 exhibit greater resistances tothe possibility of tube failure by reducing the probability of fracturein the molybdenum alloy substrate 48/graphite disk 42 interface. Theanode target assembly 60 consists of a molybdenum alloy substrate 48that retains a significant amount of work imparted to it by forgingjoined to the graphite disk 42 using the two brazes 50, each with aservice temperature at or near 1600 degrees Celsius.

Referring now to FIG. 4, a logic flow diagram for producing the anodetarget assembly 60 prior to attachment to the stem 24 is shown. First,in Step 100, a pure metal braze 44 is selected for brazing the graphitedisk 42 to the molybdenum alloy sheet 40 to form the plated graphitesubassembly 46. For a pure titanium braze, proceed to Step 110. For apure zirconium braze, proceed to Step 120.

In Step 110, the pure titanium braze is placed between the graphite disk42 and molybdenum sheet 40. The components are then placed into a highvacuum furnace at between 1700 and 1800 degrees Celsius for 3-4 minutesto melt the titanium braze and diffuse a portion of the molybdenum intothe titanium braze.

The components are then cooled, wherein the titanium braze materialsolidifies to form an integral bond between the graphite disk 42 and themolybdenum sheet 40 to form a plated graphite subassembly 46. The logicthen proceeds to Step 130.

In Step 120, the pure zirconium braze is placed between the graphitedisk 42 and molybdenum sheet 40. The components are then placed into ahigh vacuum furnace at between 1500 and 1600 degrees Celsius for 3-4minutes to melt the zirconium braze and diffuse a portion of themolybdenum into the zirconium braze. The components are then cooled,wherein the zirconium braze material solidifies to form an integral bondbetween the graphite disk 42 and the molybdenum sheet 40 to form aplated graphite subassembly 46. The logic then proceeds to Step 130.

In Step 130, a highly specialized alloy braze 50 is selected for brazingthe plated graphite subassembly 46 to the molybdenum alloy substrate 48to form the anode target assembly 60. For Ti—Cr—Be braze, proceed toStep 140. For Ti—Si braze, proceed to Step 150.

In Step 140, the Ti—Cr—Be braze sheet is placed between the platedgraphite subassembly 46 and the molybdenum alloy substrate 48 and brazedin a vacuum furnace at approximately 1110 degrees for 2-3 minutes. Athree-step vacuum furnace diffusion heat treating process immediatelyfollows the brazing step. This three step heat treating process consistsof an eight hour hold at approximately 980 degrees Celsius followed by asecond eight hour hold at 1095 degrees Celsius followed by a final fortyhour hold at approximately 1120 degrees Celsius. Remelt of the brazejoint will be approximately 1700 degrees Celsius after the three stepdiffusion heat treating process. The anode target assembly 60 is thencooled and removed from the furnace. The logic then proceeds to Step160.

In Step 150, the Ti—Si braze sheet is placed between the plated graphitesubassembly 46 and the molybdenum alloy substrate 48 and brazed in avacuum furnace at approximately 1370 degrees for 2-3 minutes. A singlestep vacuum furnace diffusion heat treating process that consists of atwo-hour hold at approximately 1200 degrees Celsius immediately followsthe brazing step. Remelt of the braze joint will be approximately 1700degrees Celsius after the diffusion heat treating process. The anodetarget assembly 60 is then cooled and removed from the furnace. Thelogic then proceeds to Step 160.

In Step 160, the anode target assembly 60 is final machined and readyfor assembly to the bearing shaft 20.

While the invention has been described in connection with one or moreembodiments, it should be understood that the invention is not limitedto those embodiments. On the contrary, the invention is intended tocover all alternatives, modifications, and equivalents, as may beincluded within the spirit and scope of the appended claims.

What is claimed is:
 1. A method for joining a molybdenum alloy substrateto a graphite disk in a rotating anode X-ray tube target assembly, themethod comprising the steps of: brazing a thin molybdenum alloy sheet tothe graphite disk using a pure metal braze to form a plated graphitesubassembly; and brazing said plated graphite subassembly to themolybdenum alloy substrate using a highly specialized braze alloy. 2.The method of claim 1, wherein the step of brazing a thin molybdenumalloy sheet to the graphite disk comprises the step of brazing a thinmolybdenum alloy sheet to the graphite disk using a pure titanium brazeto form a plated graphite subassembly.
 3. The method of claim 1, whereinthe step of brazing a thin molybdenum alloy sheet to the graphite diskcomprises the step of brazing a thin molybdenum alloy sheet to thegraphite disk using a pure zirconium braze to form a plated graphitesubassembly.
 4. The method of claim 1, wherein the step of brazing saidplated graphite subassembly to the molybdenum alloy substrate comprisesthe step of brazing said plated graphite subassembly to the molybdenumalloy substrate using a highly specialized braze alloy, wherein saidhighly specialized braze alloy has a melt temperature below therecrystallization temperature of said molybdenum alloy substrate and aremelt temperature after brazing above the recrystallization temperatureof said molybdenum alloy substrate.
 5. The method of claim 1, whereinthe step of brazing said plated graphite subassembly to the molybdenumalloy substrate comprises the step of brazing said plated graphitesubassembly to the molybdenum alloy substrate using a highly specializedbraze alloy, wherein the composition of said highly specialized brazealloy contains approximately 72 parts by weight titanium, 25 parts byweight chromium, and 3 parts by weight beryllium.
 6. The method of claim1, wherein the step of brazing said plated graphite subassembly to themolybdenum alloy substrate comprises the step of brazing said platedgraphite subassembly to the molybdenum alloy substrate using a highlyspecialized braze alloy, wherein the composition of said highlyspecialized braze alloy contains approximately 91.5 parts by weighttitanium and 8.5 parts by weight silicon.
 7. A method for reducing tubefailure due to graphite disk fracture in a rotating anode X-ray tubetarget assembly used in computed tomography applications, the methodcomprising: introducing a thin molybdenum alloy sheet between amolybdenum alloy substrate and the graphite disk; brazing one side ofsaid thin molybdenum alloy sheet to the graphite disk to form a platedgraphite subassembly; subsequently brazing the other side of said thinmolybdenum alloy sheet to the molybdenum alloy substrate to form ananode target assembly; and subsequently heat-treating said anode targetassembly.
 8. The method of claim 7, wherein the step of brazing one sideof said thin molybdenum alloy sheet to the graphite disk to form aplated graphite subassembly comprises the steps of: coupling a puretitanium braze between the graphite disk and a thin molybdenum sheet toform a molybdenum graphite subassembly; heating said molybdenum graphitesubassembly for between approximately three and four minutes at between1700 and 1800 degrees Celsius to form a plated graphite subassembly. 9.The method of claim 7, wherein the step of brazing one side of said thinmolybdenum alloy sheet to the graphite disk to form a plated graphitesubassembly comprises the steps of: coupling a pure zirconium brazebetween the graphite disk and said thin molybdenum sheet; and heatingsaid pure zirconium braze, the graphite disk and said thin molybdenumsheet for between approximately three and four minutes at between 1500and 1600 degrees Celsius to form a plated graphite subassembly.
 10. Themethod of claim 7, wherein the step of subsequently brazing the otherside of said thin molybdenum alloy sheet to said molybdenum alloysubstrate comprises the step of subsequently brazing the other side ofsaid thin molybdenum alloy sheet to said molybdenum alloy substrate witha highly specialized alloy braze having a melt temperature below therecrystallization temperature of said molybdenum alloy substrate and aremelt temperature after brazing above the recrystallization temperatureof said molybdenum alloy substrate.
 11. The method of claim 7, whereinthe steps of subsequently brazing and subsequently heat-treatingcomprises the steps of: subsequently brazing atitanium-chromium-beryllium braze between the other side of said thinmolybdenum sheet and the molybdenum alloy substrate at approximately1100 degrees Celsius for approximately two to three minutes to form ananode target assembly; subsequently heat treating said anode targetassembly for approximately eight hours at approximately 980 degreesCelsius; subsequently heat treating said anode target assembly forapproximately eight hours at approximately 1095 degrees Celsius; andsubsequently heat-treating said anode target assembly for approximatelyforty hours at approximately 1120 degrees Celsius.
 12. The method ofclaim 7, wherein the step of subsequently brazing and subsequentlyheat-treating comprises the steps of: subsequently brazing atitanium-silicon braze between the other side of said thin molybdenumsheet and the molybdenum alloy substrate at approximately 1370 degreesCelsius for approximately two to three minutes to form an anode targetassembly; and subsequently heat-treating said anode target assembly forapproximately two hours at approximately 1200 degrees Celsius.